Monoclonal antibodies directed against leukemia virus polypeptides found at the surface of leukemic mouse cell and against a T cell differentiation antigen (Thy-1) will be covalently coupled to ricin. Ricin is an extremely toxic inhibitor of eucaryotic cells and prevents protein synthesis by enzymatically modifying the ribosome. Internalization of a single molecule is sufficient to result in cell death. The ability of the conjugate to inhibit the growth of cultured cells will be correlated with the presence or absence of the target antigen. The efficacy of conjugates made by different routes using various bifunctional reagents will be examined. The ability of the conjugate to protect mice against challenge with leukemia cells will be tested.